Self-consistent calculation of the phonon-libron spectrum in α-N2

The temperature dependence of the librational and translational lattice modes in α nitrogen is calculated on the basis of two approximate treatments. The random phase approximation is shown to be unsatisfactory in predicting the temperature dependence of the librational frequencies. The self-consistent phonon approximation is found to be a definite improvement over the random phase approximation. The first model uses quadrupolar interactions alone, the second an atom-atom potential. Although the atom-atom potential is moderately successful in fitting observed properties, the evidence for the necessity of a more complete potential, containing quadrupolar as well as short range and long range anisotropic terms, is discussed.